The invention relates to a diagnostic radiology apparatus for producing layer images of a radiography subject, comprising a patient support, a measuring arrangement including a radiation source for irradiating the radiography subject from various directions, the radiation source providing a fan-shaped radiation beam penetrating the layer to be examined, whose extent perpendicular to the layer plane is approximately equal to the layer thickness, and also including a radiation receiver comprised of a series of detectors, which delivers electric output signals corresponding to the measured radiation intensity, a rotating frame on which the radiation source and the radiation receiver are mounted, and a measured value converter with a display unit, for converting the output signals of the radiation receiver into an image of the examined layer.
A diagnostic radiology apparatus of this type, a so-called computer tomograph, is described, for example, in the German OS No. 27 41 732 (U.S. Pat. No. 4,193,001 issued Mar. 11, 1980). The patient is here irradiated from different projections. A computer calculates, from the output signals of the radiation receiver, the attenuation values of specific image points arranged in a matrix. Thus, in the case of the known computer tomograph, one data set is produced per projection. In the computer, after a preprocessing operation, which, in addition to calibration also provides the logrithmation, every data set is subjected to a convolution calculation and then projected back; i.e., added into an image matrix as an addition which, at the end of a scan cycle, and hence also of a computer run, contains the attenuation values of the image points. These attenuation values can then be reproduced optically on a display unit.
If, in the case of the known computer tomograph, a detector, with regard to its transient response, exhibits a deviation from the transient response of the other detectors, then, by means of the latter detector, not the correct value, corresponding to its beam path in the fan-shaped beam configuration, but a false value, is projected into the image matrix. Since the beam path associated with the detector is always the tangent of a circle which is concentric relative to the system center, erroneous information is also circularly built up in the image matrix. Due to the convolution the erroneous signals also have a slight effect on the detector-output signals associated with adjacent beam paths. Thus, in the case of a defective detector element, circular artifacts appear in the image, whereby the connecting line: focus-detector element--projected into the image--is the tangent of a circle.